Motor vehicles are well known and are often fitted with automatic transmissions of various types in order to improve comfort and reduce driver workload. It is common for a conventional automatic transmission to include a hydrodynamic torque converter to facilitate launch, or the process of pulling away from a stationary condition. The inherent behaviour of a torque converter leads it to transmit some engine torque to the vehicle when the vehicle is stationary, the transmission has a gear engaged and the engine is at its idle speed. This leads to the vehicle accelerating from rest to a low speed when the brakes are released on a level road surface. This behaviour is known as creep and allows the driver to easily manoeuver the vehicle when parking using only the brakes to control the vehicle speed. Other automatic transmission configurations may provide a similar function by the control of clutches or electric motors. These transmissions could include, but are not limited to automated manual transmissions, automated clutch transmissions, dual clutch transmissions and hybrid or electric transmissions. In this case the transmission provides the function in order to make a parking or low speed manoeuvring more convenient for the driver.
A motor vehicle will generally be fitted with two braking systems, the foundation brakes and parking brake. The foundation brakes are normally applied by the use of a pedal and are used to slow or stop the vehicle in normal driving manoeuvres. The parking brake is normally applied by the use of a lever, a latching pedal or electrical switch and is used when the vehicle is stationary in order to hold the vehicle and allow the driver to leave. It is common for these systems to be electronically assisted or controlled to provide enhanced features such as antilock braking, traction control, drive away release and improved parking force with low lever force. In order to provide these enhanced functions the braking system requires a number of sensors such as wheel speed sensors, vehicle longitudinal acceleration sensors and inclination sensors. The data from these sensors is often available on a vehicle communication network such as a controller area network (CAN) bus and may be used by other controllers, reducing duplication of sensors or improving detection and failsafe behaviour in the event of a sensor failure.
It is known that a vehicle equipped with an automatic transmission will creep as described above on a level surface. On a slight slope the creep will reduce and as the slope increases the vehicle will stop. If the vehicle encounters a steeper slope it will roll backward unless the driver actuates the braking system or accelerates. In order to prevent rolling back on a slope it is known to configure the braking system to maintain pressure in the braking system until the powertrain provides sufficient tractive effort to accelerate the vehicle up the slope. This is commonly known as a ‘hill hold’ function and makes pull away on a hill more convenient for the driver. Even with the hill hold function the driver will have to apply more accelerator pedal to pull away on a steep slope than on a gentle slope.
It is known that in order to pull away on a steep slope the driver will have to depress the accelerator pedal further than on a gentle slope in order to generate more engine torque and increase the available tractive effort. This is observed as a region of ‘dead’ travel where the pedal is moving but having no effect on the acceleration of the vehicle.
It is an aim of the present invention to improve control of the powertrain during a hill start so the vehicle will behave in a more consistent manner whatever slope is encountered. The invention will increase the creep torque available as slope increases and reduce dead travel from the accelerator pedal.